Marks completion of 6 of 10 phases in FAJAR_LANG_PERFECTION_PLAN:
- P0  audit + plan
- P1  hygiene batch
- P2  test coverage residuals
- P3  feature-gate matrix audit
- P4  soundness probes (C1 polonius / C2 error codes / C3 fuzz)
- P5  LSP + IDE quality (D1 editor packages / D2 lsp_v3 / D3 miette)

P4 + P5 contribution: ~200 new tests across 7 new test files. Zero
production code changes. P4 alone closed in ~4.5h vs 30-50h plan
(-85%); P5 in ~1.5h vs 24-32h (-94%).

Quality gates at tag time:
- 7,626 lib tests pass at --test-threads=64
- clippy --tests --release -- -D warnings → exit 0
- cargo fmt --check → exit 0
- python3 scripts/audit_error_codes.py --strict → exit 0; gap=0
- 0 production unwraps; 0 doc warnings

Remaining phases: P6 examples + docs depth, P7 distribution unblock,
P8 LLVM O2 miscompile, P9 closeout synthesis.

Findings docs:
- docs/FAJAR_LANG_PERFECTION_PHASE_4_FINDINGS.md
- docs/FAJAR_LANG_PERFECTION_PHASE_5_FINDINGS.md

Plan: docs/FAJAR_LANG_PERFECTION_PLAN.md